The present invention relates generally to the field of marine seismic surveying. More particularly, in one or more embodiments, this invention relates to methods and systems that include using electrically powered seismic sources that are distributed at spaced apart locations.
Techniques for marine surveying include marine seismic surveying, in which geophysical data may be collected from below the Earth's surface. Seismic surveying has applications in mineral and energy exploration and production to help identify locations of hydrocarbon-bearing formations. Marine seismic surveying is typically performed using one or more “streamers” towed below or near the surface of a body of water. The streamers are typically cables that include a plurality of sensors disposed thereon at spaced apart locations along the length of the cable. The sensors may be configured to generate a signal that is related to a parameter being measured by the sensor. A seismic source may also be towed through the water by the same or a different vessel. At selected times, the seismic source may be actuated to generate, for example, seismic energy that travels downwardly through the water and into the subsurface rock. Seismic energy that interacts with interfaces, generally at the boundaries between layers of rock formations, may be returned toward the surface and detected by the sensors on the streamers. The detected energy may be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
Most of the seismic sources employed today in marine seismic surveying are of the impulsive type, in which efforts are made to generate as much energy as possible during as short a time span as possible. The most commonly used of these impulsive sources are air guns that typically utilize compressed air to generate a sound wave. Other examples of impulse-type sources include explosives and weight-drop impulse sources. Another type of seismic source that can be used in seismic surveying includes vibrator sources, including hydraulically powered sources, electro-mechanical vibrators, electrical marine vibrators, and sources employing piezoelectric or magnetostrictive material. Vibrator sources typically generate vibrations through a range of frequencies in a pattern known as a “sweep” or “chirp.”
While air guns are the most commonly employed sources in seismic surveying today, there are several limitations with these types of sources. Air guns are typically supplied with air from compressors on the vessel through a line, commonly referred to as an “umbilical.” Due to pressure losses in the umbilicals, there is a limit to the practical length at which the air guns can be deployed from the vessel, normally about 300 meters. Moreover, the size of the umbilicals (normally about 60-70 mm in diameter and about 300 meters in length) needed for conveying sufficient air to the guns also limits the lateral offset that can be used for the air guns due to drag on the umbilicals. Thus, there is a need for improved seismic surveys using other types of seismic sources without the limitations of air guns.